Acoustic resonators (also called “acoustic filters”) can be used for filtering high-frequency signal waves. Using a piezoelectric material as a vibrating medium, acoustic resonators operate by transforming an electrical signal wave that is propagating along an electrical conductor into an acoustic signal wave that is propagating via the piezoelectric material. The acoustic signal wave propagates at a velocity having a magnitude that is significantly less than that of the propagation velocity of the electrical signal wave. Generally, the magnitude of the propagation velocity of a signal wave is proportional to a size of a wavelength of the signal wave. Consequently, after conversion of an electrical signal into an acoustic signal, the wavelength of the acoustic signal wave is significantly smaller than the wavelength of the electrical signal wave. The resulting smaller wavelength of the acoustic signal enables filtering to be performed using a smaller filter device. This permits acoustic resonators to be used in electronic devices having size constraints, such as cellular phones and smart watches.
Surface acoustic wave (“SAW”) filters (also called “SAW resonators”) are a type of acoustic resonator that includes a system of two groups of inter-digital transducers (“IDTs”), with each of the two groups arranged between reflection gratings (also called “reflectors”). Each IDT includes a first bus bar (also called a “rail”) coupled to either an input terminal or an output terminal, and a second bus bar, spaced from the first bus bar, connected to ground. The first bus bar couples together a first set of fingers extending from the first bus bar toward the second bus bar. The second bus bar couples together a second set of fingers extending from the second bus bar toward the first bus bar such that the second set of fingers extend between fingers of the first set of fingers. An overlap of the first set of fingers and the second set of fingers functions as multiple capacitors providing electric fields across a surface of the piezoelectric material. The length of the overlap is called the aperture of the IDT. The electric fields provided by the overlap cause the piezoelectric material to vibrate with a frequency based at least in part on a distance between each finger extending from the first bus bar and adjacent fingers extending from the second bus bar. This distance between fingers of an IDT is called a pitch. The frequency at which the piezoelectric material vibrates is a self-resonance (also called a “main-resonance”) frequency of the IDT. An additional resonance is formed from a cavity resonance based in part on a distance between adjacent IDTs and chirping regions of the adjacent IDTs.
Desirable qualities of a SAW filter include a high quality factor to indicate effective selectivity of a desired frequency, low energy loss, and a small chip-footprint. Conventional SAW filters use chirping between IDTs to decrease acoustic losses, and thus increase a quality factor. Additionally, chirping is used to facilitate a smooth transition between IDTs of the SAW filter, improving filtering accuracy. Chirping is facilitated by arranging fingers of an IDT, generally those at a longitudinal end of the IDT, at a pitch that varies from the main region of the IDT. More interfaces between chirped regions of adjacent IDTs generally increase a quality factor of the SAW filter. Examples of chirping configurations are described in U.S. Pat. Nos. 6,420,946, 7,042,132, 6,894,588, and 6,972,643, which are incorporated herein by reference.
Conventional SAW filters may also have an increased aperture to increase a capacitance of the SAW filter, and thus reduce acoustic energy losses during operation. However, increasing an aperture of the SAW filter also increases insertion energy losses and increases a chip-footprint of the SAW filter.
This background provides context for the disclosure. Unless otherwise indicated, material described in this section is not prior art to the claims in this disclosure and is not admitted to be prior art by inclusion in this section.